Dealkylation of aromatic hydrocarbons



U ited States Patent DEALKYLATION 0F AROMATIC HYDROCARBONS Harry C lough, Norton-on-Tees, England, assignor to Imperial Chemical'lndustries Limited, a corporation of GreatBritain No Drawing. Application June 29, 1951, Serial No. 234,436

Claims priority, application Great Britain July 17, 1950 6 Claims. (Cl. 260-672) tion and coking of coal, from coal tars, from fractions of certain petroleums and from petrols obtained by the treatment of petroleum or its fractions, the individual aromatic hydrocarbons being obtained from such sources, for, example by fractional distillation, solvent extraction or a combination of such processes. Often such materials from which the individual aromatic hydrocarbons are to be isolated do not contain a high proportion of the more valuable lower aromatic hydrocarbons.

An object of the present invention is to provide a process whereby an enhanced source of individual lower aromatic hydrocarbons of improved purity is obtained. A further objectis to provide a process forthe treatment of individual aromatic hydrocarbons such as toluene or a xylene to produce lower aromatic hydrocarbons. A particular object of the present invention is to produce enhanced quantities of high grade benzene, toluene and xylenes from materials containing or consisting of higher aromatic hydrocarbons.

In its broader aspects the process of the present invention comprises the dealkylation of aromatic hydrocarbons.

According to the present invention therefore there is provided a process for the dealkylation of monocyclic aromatic hydrocarbons which comprises treating a monocyclic aromatic hydrocarbon at a temperature above 500 C. and at a pressure higher than 100 atmospheres gauge with hydrogen in the presence of a catalyst comprising an aluminum phosphate.

While in general the alkyl groups to be removed may contain several carbon atoms, for example ethyl and propyl groups, we have found the process of the present invention particularly suitable for the removal of methyl groups. Particularly satisfactory results have been obtained for example in the dealkylation of toluene, xylenes, ethyl benzene and trimethylbenzenes. Furthermore, the process of the invention is suitable for the treatment of hydrocarbon mixtures containing monocyclic aromatic hydrocarbons, examples of such mixtures being products obtained by extracting petroleum products such as petrol and kerosene with liquid sulphur dioxide, oils, for example petrols obtained by the conversion of petroleum and its fractions and oils recovered during the gasification and coking of coal and extracts of fractions from the distillation of coal tar. In addition to paraffins and naphthenes, the hydrocarbon mixtures may also contain olefines and diolefins. Good results have been obtained with fractions of cracked petrols boiling within the range 125 to 150 C. and in the range of 150 to 175 C. When treating fractions of cracked petrols results indicate that in addition to the dealkylation some dehydrogenation and cyclization may have oc- "ice curred leading to the formation of further aromatic hydrocarbons. In this specification, the wor d conversion as applied to petroleum and its fractions is intended to mean those processes by which petroleum and its frac: tions are treated to obtain petrols, naphthas, kerosenes and other valuable products, the processes including cracking and catalytic treatments to cause dehydrogenation and isomerization.

Catalysts suitable for use according to the present invention may consist of aluminum orthophosphate (AlPO4) for example in the form of pellets, or this compound may be admixed with or supported-on so called cracking catalysts comprising alumina, silica or mixtures or compounds thereof. The aluminum phosphate may be admixedor supported on the so called earths, i. e the aluminum silicates, for example, those sold under the trade name Superfiltrol.

A Wide range of temperatures above 500 C. may be used. At temperatures below 500 C. saturation of aromatic hydrocarbons tends to occur and this tendency in.- creases as the temperature is decreased below 500 C. Furthermore, at temperatures below 500 C. there is a tendency to disproportionation of alkyl groups leading to the formation of undesired aromatic hydrocarbons of higher molecular weight. As the temperature is increased there is a tendency for cracking reactions to occur accompanied by polymerization of the cracked products, and for this reason it is preferable to 'operate the process of the present invention in the temperature range500 to 600 C. More preferably the process is operated at temperatures in the range of 540 C. to 570 C.

, The pressure used in the process of the present invention will depend in general on the type ofaromatic hydrocarbons desired. It is desirable however to avoid using pressures lower than atmospheres gauge as there may be a tendency to disproportionation. We have found that pressures in the range 200 to 300 atmosphere gauge give particularly satisfactory results. As the pressure is increased it is desirable to increase the temperature of operation, to avoid saturation of the aromatic hydrocarbons. In general, in commencing. operation of the process, the desired pressure will be attained by the introduction of substantially pure hydrogen into the apparatus and it will thereafter be maintained by the introduction of similar gas as required.

While the process of the present invention may be carried out by introducing the reactants and catalyst into an autoclave, and keeping them there until reaction has proceeded to a satisfactory extent, it is preferable to carry it out in a continuous process, that is in one in which the reactants are passed over the catalytic material maintained at a suitable temperature. It has been found that in such a continuous process the rate of passage of the reactants over the catalyst has an important effect on the composition of the product, as will be evident from the record, given hereinafter, of experiments carried out according to the invention.

As the process of the present invention in general is accompanied by the formation of carbonaceous material, it may be desirable from time to time to regenerate the catalyst, for example by the passage thereover of an oxygen containing gas such as air and/or steam while maintaining the catalyst at elevated temperature. This regeneration step may be carried out, for example at intervals of 300 hours when working a continuous process.

It will be understood that the products obtained from the operation of the process may be treated by any convenient means for the isolation of the individual monocyclic aromatic hydrocarbons, for example simple fractional distillation, azeotropic distillation and crystallization or a combination of such methods may be used.

Example A series of experiments was carried out in which a mixture of xylenes, together with hydrogen, was passed over pelleted aluminum orthophosphate, prepared as hereinafter described, maintained at a temperature of 550- C. the pressure being maintained at 260 atmospheres gauge. The liquid products were collected and analyzed for their contents of benzene, toluene and xylenes. The results obtained with different rates of feeding the xylene mixture were as follows, the feed rates of the xylene mixture being expressed as liters of liquid xylene mixture per literof catalyst per hour..

Analysis of Product: Percent Weight Feed Rate Benzene Toluene Xylenes of alkyl groups of monocyclic aromatic hydrocarbons containing at least one substituent alkyl group which comprises the step of treating a monocyclic aromatic hydrocarbon containing at least one substituent alkyl group with hydrogen at a temperature higher than 500 C. and a pressure higher than 100 atmospheres gauge in the presence of a catalyst consisting essentially of an aluminum phosphate.

2. A process as recited in claim 1 in which the temperature is within the range of from 500 C. to 600 C. and the pressure is within the range of from 200 to 300 atmospheres gauge.

3. A process for the dealkylationby complete removal of alkyl groups of monocyclic aromatic hydrocarbons containing at least one substituent alkyl group which comprises the step of treating a monocyclic aromatic hydrocarbon containing at least one substituent alkyl group with hydrogenat a temperature higher than 500 C. and a pressure higher than 100 atmospheres gauge in the presence of a catalyst which consists essentially of an aluminum phosphate together with a material selected from the group consisting of alumina, silica, aluminum silicate and mixtures thereof.

4. A process as recited in claim 3 in which the temperature is within the range of from 500 C. to 600 C. and the pressure is within the range of from 200 to 300 atmospheres gauge.

5. A process as recited in claim 1, wherein the aromatic hydrocarbon is selected from the group consisting of toluene, xylene, ethyl benzene and trimethyl benzene.

6. The process of claim 5, wherein the aromatic hydrocarbon is xylene.

References Cited in the file of this patent UNITED STATES PATENTS 2,349,827 Mattox May 30, 1944 2,396,76l Tilton Mar. 19, 1946 2,438,570 MatlOX Mar. 30, 1948 

1. A PROCESS FOR THE DEALKYLATION BY COMPLETE REMOVAL OF ALKYL GROUPS OF MONOCYCLIC AROMATIC HYDROCARBONS CONTAINING AT LEAST ONE SUBSTITUENT ALKYL GROUP WHICH COMPRISES THE STEP OF TREATING A MONOCYCLIC AROMATIC HYDROCARBON CONTAINING AT LEAST ONE SUBSTITUENT ALKYL GROUP WITH HYDROGEN AT A TEMPERATURE HIGHER THAN 500* C. AND A PRESSURE HIGHER THAN 100 ATMOSPHERES GUAGE IN THE PRESENCE OF A CATALYST CONSISTING ESSENTIALLY OF AN ALUMINUM PHOSPHATE. 